Note: This is not the cheaper f/1.8 version. This lens gives you an extra half stop for low light situations. Unless you need it for that purpose, there is no real need to buy this over the f/1.8.
If you shoot at dusk, indoors or in places where you can't use flash, the 1.4 is invaluable.
I've ended up with three different versions of this lens - the AF-S arrived today, an AF-D arrived for my son earlier in the week and the one for sale is the previous version to that - the AF version. Pretty much the same as the AF-D, but a slightly different focus ring design amongst other small cosmetic differences. A picture up of both lenses next to each other below so you can compare.